1. Field of the Invention
This invention relates to cryogenic cleaning systems, and more particularly, to a system and method for controlling the humidity within the workspace of a cryogenic aerosol spray cleaning system by circulating the workspace atmosphere through a dehumidifier to eliminate the need to purge the workspace prior to each cleaning cycle.
2. Description of the Prior Art
Precision cleaning using solid, liquid, or gaseous carbon dioxide, or other cryogenic based cleaning materials and methods, including mixed carbon dioxide 20 phases, solid or liquid argon sprays, liquid nitrogen sprays, and even ice (H2O), or combinations of these materials and methods, has been disclosed by patents and prior art publications. The cryogenic spray cleaning technologies disclosed by the prior art have evolved in response to many commercial, industrial, and practical concerns, including environmental concerns, and the need for better and more effective cleaning methods for both particle and organic-based contamination.
Carbon dioxide snow cleaning was first disclosed by S. A. Hoenig around 1985. The process typically involves cleaning, using a source of fluid (i.e., liquid or gaseous) carbon dioxide provided at a certain enthalpy condition (i.e., temperature and pressure). Such liquid carbon dioxide (or gaseous carbon dioxide if proper adiabatic conditions are met by the nozzle design) is passed at high velocity through an orifice of a spray nozzle. Upon exiting the orifice, a stream of dry ice particles having varying sizes and densities and traveling at varying velocities is directed at a workpiece for removal of contaminates deposited on a surface thereof. The stream of dry ice particles may be combined with CO2 gas, nitrogen or other dry gases to boost 10 the dry ice velocity.
Another form of cryogenic cleaning uses macroscopic CO2 pellets and was first described by Rice et al. in U.S. Pat. No. 3,676,963, and by Fong in U.S. Pat. No. 4,038,786. This snow cleaning method feeds dry ice pellets into a dry carrier gas stream such as nitrogen or dry compressed air, at a pressure typically in the range of between 40 and 300 psi and possibly greater. The dry ice pellets are accelerated toward the surface of a workpiece at such high velocity that even thick coatings such as paint can be removed.
One limitation in cryogenic spray cleaning methods is the fact that extremely cold streams (e. g., typically xe2x88x9260xc2x0 C. for carbon dioxide snow cleaning) of cleaning medium are applied to the surface of the workpiece being cleaned. Consequently, when the cleaning processes are conducted at room temperature, or in any unsealed and uncontrolled environment, condensation can form on the workpiece surface being cleaned, or on the spray nozzle. The moisture condensation, present as water, frost, or ice, interferes with and impedes the cleaning process. Prior art attempts to eliminate moisture include direct heating, heating with blanketing gases or heating only specific portions of the surface of the workpiece, insulating the cleaning chamber, cleaning in vacuum environments, purging with nitrogen, air or other dry inert gases, purging using specially constructed chambers, and other methods utilized to produce dry environments.
Patents directed to clean and dry chambers for cryogenic spray cleaning systems and methods typically include vacuum chambers and/or require purging a clean dry box with a dry inert gas. For example, U.S. Pat. No. 4,631,250 to Hayashi was the first to mention indirectly the need for a sealed chamber for a cleaning system that had mixed CO2 and nitrogen for cleaning a wafer surface. The sealed chamber included a vacuum exhaust line for removal of contaminants. Of course, a vacuum environment also assists in moisture reduction.
Another attempt to control the humidity in the cleaning chamber of a cryogenic cleaning system is disclosed in U.S. Pat. No. 5,316,560 to Krone-Schmidt et al. This patent discloses purging an enclosed space (i.e., a cleaning chamber) with dry nitrogen gas to control the humidity within the chamber. Essential to this system is the chamber within a chamber design having a purged airlock between the enclosed interior cleaning chamber and the exterior of the system. The system disclosed by this reference requires lengthy purge times to dry out the cleaning chamber before initiation of a cleaning cycle, and may therefore not be practical for cleaning a large workpiece in a large volume cleaning chamber.
In U.S. Pat. No. 5,315,793 to Peterson et al., an apparatus for precision cleaning using CO2 snow or other cryogenic sprays is disclosed. The apparatus disclosed is intended only as a final cleaning station and has some design features in common with U.S. Pat. No. 5,316,560. The dryness of the sealed chamber is maintained by vacuum or by an external inert gas purge. The preferred method for maintaining a dry environment involves pumping out and then back-filling the chamber with a dry gas after each cleaning cycle, i.e., to purge the cleaning chamber.
Cryogenic argon spray cleaning developed as an alternative to CO2 snow cleaning. Cryogenic argon spray cleaning, and, in parallel, cryogenic nitrogen spray cleaning, were first mentioned in U.S. Pat. No. 5,062,898, and later in U.S. Pat. No. 5,294,261, and still further developed in U.S. Pat. No. 5,209,028, all to McDermott et al. These patents disclose the nozzles and cleaning stations for cryogenic argon spray cleaning, and methods and procedures for ensuring a dry cleaning station based upon a flush gas for removal of contaminants.
Further developments in cryogenic argon spray cleaning are disclosed in U.S. Pat. No. 5,486,132 to Cavalier et al. and in U.S. Pat. No. 5,366,156 to Bauer et al., wherein argon spray cleaning methods and apparatus are extended to include cryogenic nitrogen or mixed argon and nitrogen. These patents also clearly discuss means to reduce and prevent moisture condensation from forming on the apparatus, nozzles, argon or nitrogen lines, and on the workpiece surface to be cleaned.
Moisture elimination methods discussed and claimed included purge means, such as dry gas purge methods with purge ports and purge gas sources, providing a vacuum about the cryogenic argon lines and nozzle, providing thermal insulation and barriers and positive pressure within the enclosures, and other suggestions unrelated to the present invention.
A means for eliminating moisture in a CO? dry ice pellet system cleaning apparatus is disclosed in U.S. Pat. No. 5,651,723 to Bjornard et al. The apparatus disclosed therein includes separate load locks for loading and unloading a workpiece, a cleaning chamber between the two load locks, and the necessary equipment to provide airflow through the cleaning chamber and apparatus and to purge the cleaning chamber. Dry compressed air is required for accelerating the dry ice pellets only. A dry environment was ensured in the load locks by purge methods and all chambers were kept at positive pressures to keep moisture out.
Systems and apparatus for maintaining a dry and clean manufacturing environment abound within the electronic, chemical and pharmaceutical industries. However, none of the known systems, apparatus, methods, processes, etc. use a dehumidifier in conjunction with a cryogenic spray cleaning process to control the humidity within a cleaning chamber, thereby obviating the need for time-consuming and expensive purging of the cleaning chamber. For large cleaning chambers, purging as a means for controlling humidity within a cleaning chamber becomes cost-prohibitive, and therefore, commercially impractical.
The present invention overcomes the shortcomings of the prior art by providing a system and method for controlling the humidity within the workspace of a cryogenic aerosol spray cleaning system by circulating the workspace atmosphere through a dehumidifier placed in the airflow path of the cleaning system. The present invention obviates the need to purge the workspace atmosphere at any time during a cleaning cycle and further eliminates the need to provide a sealed cleaning chamber. The present invention provides for rapid moisture removal from the workspace atmosphere and for dehumidifying the entire atmosphere in a relatively short period, typically approximately two minutes. This, in turn, leads to increased productivity for the cryogenic cleaning system.
The present invention provides a system and method for controlling the humidity within the workspace of a cryogenic aerosol spray cleaning system by circulating the workspace atmosphere through a dehumidifier to eliminate the need to purge the workspace prior to each cleaning cycle. The system and method of the present invention are especially well-suited, economical, and a practical necessity for cleaning of large workpieces, where a dry air or inert gas purge of a large volume cleaning chamber, as required by prior art methods and apparatus, would be impractical.
Prior art systems and methods for controlling the humidity in a cryogenic cleaning system require time-consuming inert gas purges, expensive vacuums or other means, most all in conjunction with load locks. On the other hand, the present invention eliminates the need to purge the cleaning chamber workspace and produces a dry, cleaning environment quickly and economically. The present invention also eliminates the need to provide a sealed cleaning chamber and to provide load locks at the input and output sides of the cleaning chamber; all directed to maintaining a clean and dry environment within the cleaning chamber. The workspace atmosphere is cycled through the dehumidifier many times a minute to insure a fast dry out time.
An integral aspect of the humidity control system of the present invention is the handling of the gaseous and particulate constituents, i.e., moisture and particulate matter, present in the atmosphere within the cleaning chamber workspace. More specifically, cryogenic snow cleaning requires a virtually moisture-free and particle-free workspace atmosphere within the cleaning chamber. The present invention satisfies this requirement in a manner not disclosed or suggested by the prior art.
The humidity control system of the present invention incorporates a dehumidifier into at an airflow path defined through a cryogenic aerosol spray cleaning system including a cleaning chamber having a workspace defined therein.
The workspace atmosphere is continuously or intermittently circulated through the dehumidifier to remove moisture before, during, and/or after a cleaning cycle. An optional HEPA filter may be provided in the airflow path to remove particulate contaminants from the workspace atmosphere. A secondary airflow path may be defined through the cleaning system through which a portion of the workspace atmosphere is continuously or intermittently circulated. The dehumidifier and optional HEPA filter are located in the secondary airflow path to remove moisture and particulate contaminants from the workspace atmosphere.
The present invention is directed to a method for controlling the humidity within a cleaning chamber of a cryogenic cleaning system. The cleaning chamber has a workspace defined therein within which a workpiece may be removably placed for cryogenic cleaning during a cleaning cycle. The workspace has a partly gaseous (defined hereinafter) atmosphere therein and the cleaning system has an airflow path defined therethrough. The method of the present invention comprises the step of circulating the workspace atmosphere through a dehumidifier of an air handling system located in the airflow path for removing moisture from the gaseous part of the workspace atmosphere to reduce the humidity in the workspace atmosphere without having to purge the workspace prior to the initiation of a cleaning cycle.
The present invention is also directed to a humidity control system for a cleaning chamber of a cryogenic cleaning system. The cleaning chamber has a workspace defined therein within which a workpiece may be removably placed for cleaning during a cryogenic cleaning cycle. The workspace has a partly gaseous atmosphere therein and the cleaning system has an airflow path defined therethrough.
The cryogenic cleaning system further includes a spray nozzle located within the cleaning chamber and connected to a cleaning media source for producing a stream of solid or liquid cryogenic spray entrained by a gas phase for cryogenically cleaning the workpiece during a cleaning cycle. The humidity control system comprises a dehumidifier of an air handling system located in the airflow path and through which the workspace atmosphere is circulated for removing moisture from the gaseous part of the workspace atmosphere to reduce the humidity in said workspace without having to purge the cleaning chamber prior to initiation of a cleaning cycle.
The humidity control system of the present invention may be used in an automated or manual cryogenic cleaning system. For automated systems, motion control equipment may be included in the workspace to manipulate the workpiece (e.g., movement into and out of the workspace and manipulation therein) and cleaning components (e.g., spray nozzles). For manual cleaning systems, a glove port may be provided on a front side of the cleaning chamber through which access to the workpiece and cleaning components is provided.